1. Field of Invention
This invention relates to a Second Harmonic Generation (otherwise known as xe2x80x9cSHGxe2x80x9d) laser light source whose optical output signal can be modulated using a simple circuit and to a method of modulation for use therewith.
2. Description of the Prior Art
The surface recording density of optical disks has increased dramatically as the recording capacity of the disk has increased. This trend has led to a smaller on disk spot where the beam is focused. Accordingly, there is increased demand for the wavelength of the light source used to write and read from the disk to be as short as possible. The light sources for producing short wavelength beams include the SHG laser light source. Japanese unexamined patent application 11-26861 discloses an SHG light source which injects a fundamental wave into a nonlinear optical crystal to produce a second harmonic wave, which is a short wavelength coherent light. This type of system is shown in FIG. 1.
In FIG. 1, a distributed Bragg reflection (also known as xe2x80x9cDBRxe2x80x9d) laser diode 1 produces a fundamental wave. An SHG device 2 is supplied with an input of the fundamental wave, converts the wavelength thereof, and outputs a second harmonic wave. An external modulator 3 is supplied with the second harmonic wave from the SHG device 2, and modulates the wave, and then outputs a modulated wave.
The FIG. 1 system operates as follows. Drive circuit 4 drives DBR laser diode 1 to output a fundamental wave which is supplied to SHG device 2, which converts the wavelength thereof, and outputs a second harmonic wave. The second harmonic wave is modulated or attenuated by external modulator 3 so that the power of the wave is controlled. As described in 11-26861, SHG device 2 has such a relationship between the output power of the second harmonic wave and the input wavelength as shown in FIG. 2. As shown in FIG. 2, the output power drastically drops once the input wavelength deviates from a specific value. Accordingly, the output power is heavily dependent on the wavelength of the fundamental wave. Also, the nature of the DBR laser diode 1 is such that the wavelength of the fundamental wave also changes when the output power thereof is changed by altering the drive current.
Consequently, even when the drive current of DBR laser diode 1 is increased to raise the power of the fundamental wave, for example, in an attempt to boost the power of SHG laser light source, the wavelength of the fundamental wave may shift. The shift would result in a drop in the output power of the SHG laser light source. Accordingly, in the prior art system, it is essential that the power of the second harmonic wave outputted by the SHG device be constant and the optical output of the SHG laser light source be changed by an external modulator 3, in order to suitably modulate the optical output. Consequently, the prior art system is always provided with an external modulator 3. The external modulator 3, in turn, requires an optical system which couples the modulator to the SHG device 2. This requirement hinders use of the prior art system as a pickup device for optical disks which is increasingly required to be smaller.
Accordingly, an object of the invention is to overcome the aforementioned and other disadvantages, deficiencies and problems of the prior art.
Another object is to provide an SHG laser light source whose optical output can be modulated using a simple mechanism without having to provide an external modulator.
A further object is to provide a method of modulation for use with the SHG laser light source.